Gordon Brown would be forced to appoint a deputy who could be swiftly groomed as his successor under humiliating last-ditch plans being discussed by ministers to patch up his failing administration.

Senior figures are holding emergency talks on ways to stop what one called the 'haemorrhaging' of power from New Labour amid signs that Brown is losing control of his party. Rumours swept Westminster yesterday that one senior cabinet minister has begun raising money for a potential leadership bid, while up to 40 backbench MPs are said to be ready to back a challenge.

If Brown loses a vote on his terrorism bill in two weeks' time, as expected, that would become the catalyst. The justice secretary Jack Straw, who has already privately signalled concerns about the measures to detain terror suspects for up to 42 days, is regarded as the most likely senior figure to move against him.

Ministers plan to demand Brown appoints a deputy Prime Minister to bolster his flagging appeal, ostensibly to free him up to focus on major issues - but offering potential rivals like David Miliband, Alan Johnson or Straw the chance to shine in the role. The deputy would then be in position for a smooth transition if Brown were forced out.

'The debate is about having a deputy Prime Minister, either somebody with that title or who fulfils that role,' said a senior minister. 'It is possible the game is not actually to wield the knife (against Brown directly) but to put Gordon in a position where he has no alternative but to agree to a deputy.'

One source close to Brown said he 'may not resist too hard', even though he scrapped the deputy Prime Minister post once held by John Prescott when he entered Number 10.

Friends are urging him to defuse the danger by taking potential rivals, such as Miliband and James Purnell, publicly under his wing and talking up their futures rather than attempting to crush them - a sign that he is now too weak to confront them.

The catastrophic swing against Labour in the Crewe and Nantwich by-election has convinced many in government that Brown will lead them to defeat at the general election. One well-placed party source said fears that an alternative leader might not have time to avert defeat were receding: 'The big danger is if we lose and get annihilated, you have to spend several terms getting it back. If you can hold the party together, even if that means losing but not losing as badly, you come back faster.'

The Tories threatened last night to push for a snap election if Labour tries to change their leader again without going to the country. 'It would be quite absurd for Labour MPs to think they can foist one unelected prime minister after another on the country,' said frontbencher Chris Grayling.

Brown's inner circle is spending the weekend discussing a fightback. He is being urged to turn the threat of defeat to his advantage with a 'go for broke' strategy unleashing ideas he has previously considered too controversial to explore - modelled on the success of his bold defence last week of embryo research, late abortion and IVF treatment.

David Cameron meanwhile faces an internal party row over his commitment to green taxes in the wake of the credit crunch. The success of a Tory campaign in Crewe that attacked tax rises and the cost of living is said to have emboldened traditionalists who want him to ditch threatened levies on low-cost flights or high-polluting cars, proposed by a so-called 'quality of life' review draw up by millionaire environmentalist Zac Goldsmith and former minister John Gummer. Cameron built his reputation on green ideas, cycling to work and travelling to the Arctic to inspect melting glaciers, but his influential new communciations chief Andy Coulson is said to be sceptical about their appeal at a time when family finances are stretched.

'This is the first real challenge to politicians from all parties as to whether they are willing to sustain a commitment to more green taxes in the face of economic slowdown,' said a well-placed Conservative party source. 'The [Gummer] review is going to run into quite a critical reception because people are genuinely wondering whether this is the right time to make this kind of change.'

Gordon Brown has ditched planned rubbish taxes, while environment secretary Hilary Benn has quietly dumped plans championed by his predecessor David Miliband for a form of carbon rationing, where every citizen would be given a limited allowance of carbon to emit through activities such as driving or flying off on holiday.

A study by his officials concluded the public was not ready for the idea and a planned pilot scheme has been scrapped. A senior Conservative source said they would wait to see how the green debate unfolds, with Brown under pressure to review motoring taxes.